Within the field of offshore oil and gas production, when conveying hydrocarbons from a subsea well to a vessel at the surface, the hydrocarbons are often transported through flowlines extending along the sea floor and risers connecting the flowlines to floating production units or vessels at the surface. The flowline can extend from a single subsea wellhead, from subsea processing equipment, or from a subsea collection manifold. Tie-in systems refer to systems in which sections of subsea pipeline or flowline are connected to one another and other subsea equipment. One known tie-in system, also referred to as a “hinge over riser assembly” or HORA, is disclosed in U.S. Pat. No. 7,628,568. In the method disclosed in U.S. Pat. No. 7,628,568, a subsea flowline and riser assembly is installed on a sea floor. The method includes providing a flowline having an end connected to a hinge-over joint, and a riser having an end connected to the joint such that the axes of the riser and flowline extend substantially parallel. An end of the flowline opposite the joint is lowered to the sea floor. The end of the flowline connected to the joint is then lowered to the sea floor. The joint is connected to a foundation installed in the sea floor. The riser is rotated about the hinge-over joint such that the riser axis is substantially perpendicular to the flowline axis. A subsea jumper is connected between an opening formed in the flowline and an opening formed in the riser so that the riser is in fluid communication with the flowline. In this cited arrangement the flowline and riser are deployed linked to the joint structure but without a built-in flow path linkage, such that an additional subsea jumper is required to be fabricated pursuant to measurements taken on site. Such measurements, fabrication and installation can be time and labor intensive.
A known variation of the hinge over riser assembly employs a horizontal connection that includes a 90° load path through a flowline-riser connector which has structural and long-term fatigue challenges. These challenges require the addition of a bulky reinforcement structure surrounding the connector load path, which in turn increases the overall assembly size and weight and consequential increased operational risks during installation. Such horizontal connections also require horizontal pulling of the whole assembly, thus sliding the assembly over a surface during positioning prior to mating and locking of the assembly components to each other. The horizontal pulling with sliding introduces potential for malfunction or damage to the assembly. Potential obstructions on the sliding surfaces involved can be caused by marine growth, debris consolidation, corrosion buildup and related factors, which can create risks to the operability of the system.
There exists a need for a subsea tie-in system which would avoid or reduce the aforementioned challenges, potential for damage and potential risks.